Interior permanent magnet synchronous motors (IPMSMs) have gained widespread use in a variety of applications, including automotive applications such as electric vehicle traction motors, micro-hybrid belt-driven starter generator (BSG) motors in 48V systems, and electric power steering. IPMSMs offer many attractive characteristics including high power density, high efficiency, reluctance torque availability, and wide speed operating ranges.
An IPMSM has a characteristic base speed that must be accounted for in any control scheme. The base speed refers to a speed at which the rotor of the IPMSM cannot be accelerated by supplying additional current to the stator windings. There are many applications for IPMSMs that require the motor to be operated beyond its base speed. This is possible using a technique known as flux weakening. Flux weakening involves reduction of the permanent magnet flux linkage, i.e., the rotor field. Although this allows the rotor to spin faster, this reduction in the rotor field reduces the amount of torque generated by the motor. That is, as the IPMSM is accelerated past its base speed, the amount of torque generated by the motor decreases. The operational region below the base speed is commonly referred to as the constant torque region, whereas the region above the base speed is commonly referred to as the constant power region.
One known flux weakening technique involves voltage feedback control. See Kim et al., Speed Control of Permanent Magnet Synchronous Motor Drive for the Flux Weakening Operation, IEEE Transactions. vol. 33, no. 1, pp. 43, 48, February 1997. One drawback of this technique is that it requires relatively complicated calculation steps that can consume substantial computing resources. Furthermore, according to this technique, the maximum DC voltage of the power supply cannot be fully utilized. Instead, this technique requires a margin between the maximum DC voltage and a user-defined voltage reference output, which is required so that the current control can take effect before the motor enters the saturation region. Thus, the motor is not able to achieve its maximum speed using voltage feedback control.
Another known flux weakening technique involves a current vector control technique. See Jackson Wai, T. M. Jahns, A New Control Technique for Achieving Wide Constant Power Speed Operation with an Interior PM Alternator Machine, in Conf. Rec. IEEE IAS Ann. meet, vol. 2, pp. 807-814, 2001. According to this technique, during flux weakening, a current vector angle is controlled. One drawback of this technique is that the current vector locus can only be approached and never actually reached. This is because the current regulator loses control after voltage saturation.